Biaxially oriented polypropylene films have been employed in the roll fed label and packaging industries. One of the key product attributes that must be possessed by these films is a sufficiently low coefficient of friction (COF) to allow the film to easily slide on various machine surfaces, thus resulting in efficient processing of the films.
In addition, a second key attribute is that the surface having desired slip properties must also have sufficient adhesion properties to effectively adhere to adhesives employed in label applications.
Since the COF of polypropylene film is generally higher than that desired for effective processing in labeling applications, various additives have been incorporated into the film to reduce the COF. Two common additives employed to reduce COF are organosilicone particulate materials, such as Tospearl T120, which is supplied by G.E. Toshiba Sales Americas in Waterford, N.Y., and fatty acid amides.
When using an organosilicone particulate material such as Tospearl, the particles are typically included in a thin, coextruded surface layer of a multi-layer film. The particles tend to reduce the degree of contact between the polypropylene film and machine surface engaging the skin layer, and generally act like ball bearings to reduce the drag of the film on the machine surfaces. While this technology works well in reducing the degree of contact between the film and the machine surfaces, as well as lowering the measured COF, the resulting film surface has a roughness, or uneven contour, that has been shown to have a negative impact in certain labeling applications.
As noted above, fatty acid amides also have been employed as slip agents to reduce COF. These amides generally are compounded with the polyolefin polymer and are thus contained within the matrix of the polymer throughout the orientation process of the film. Since the polyolefin polymer and amide are generally chemically incompatible, over time the amide will migrate to the film's surface, thereby providing desired slip properties to the film. However, due to other steric, thermodynamic and practical considerations, the diffusion process often needs to be accelerated through the use of heat. Although heating the film does provide for a faster and more effective migration of the amide to the film surface, thereby lowering the COF, the exposure of the film to elevated temperatures often causes non-uniform shrinkage of the film. This can have an adverse affect on the desired flatness of the sheet, thereby creating problems during printing and or laminating processes. Moreover, excessive migration of the fatty acid amides to the surface of the film may adversely affect the adhesion properties of the film surface. This can present problems in utilizing the film in labeling applications.
Although coextruded polyolefin films have included an antistatic additive in an outer skin layer and a migratory amide in the core layer, to the best of applicant's knowledge these additives have not been employed for the purposes of reducing the COF of the skin layer against metal machine surfaces, such as metal surfaces in labeling machines, and of providing desired adhesion properties to adhesives employed in labels. Applicant is not aware of any prior art films in which the types and amounts of an antistatic additive in the outer skin and a migratory amide in the core achieve the desired COF and adhesion properties desired for adhesive label applications, as in the present invention.
In view of the state of the art, a need exists in the labeling art for a biaxially oriented polyolefin film having a desired flatness to reduce, or minimize, problems during printing and/or laminating in the manufacture of labels, desired slip properties to render the film machinable without the use of particulate materials, such as organosilicone particulate materials, in the surface layer to reduce COF, and also without the need for heating the film to enhance the rate of migration of a fatty acid amide from the core to the surface of the film for providing the desired COF, and with the desired “adhesion” (hereinafter defined) to adhesives employed in label applications, such as to hot melt adhesives. It is to such an improved multilayer polyolefin film that the present invention is directed.